This application claims priority of Korea patent Application No. 2000-57276, filed on Sep. 29, 2000.
(a) Field of the Invention
The present invention relates to a method for compensating the position of a robot using a laser measuring instrument. More particularly, the present invention relates to a method for compensating the position of a robot in which a laser measuring instrument is used to simultaneously compensate for the positioning of a welding gun, a welding robot, various jigs, etc. such that a robot teaching process time is reduced and a precision of welding point teaching for a vehicle body panel robot is enhanced.
(b) Description of the Related Art
Computer simulations are commonly used in the design of assembly processes for automobiles. That is, using computer simulations, all the processes involved in manufacture-design, manufacture and installation of manufacturing processes and lines, and the operation of the processes can be modeled before actual application. Then by running the simulation, problems can be detected and rectified before actual implementation. Accordingly, the time to prepare for production is reduced, quality is enhanced and costs are minimized. In addition to these advantages, benefits are realized through the off-line programming capabilities of simulations.
However, since in the robot programs the modeling of the situations in which the robots are placed is based on CAD data with the use of simulations, exact replication of the true conditions is not possible. That is, there results a difference between the model and the actual situation such that direct application of the robots designed using the simulations is not possible. Accordingly, it is necessary to undergo a calibration process in which these differences in the programs written off-line (i.e., differences between the designed data and actual circumstances) is compensated for, after which the calibrated programs are downloaded to a robot controller.
Such calibration of the positions of a welding gun, welding robot, various jigs and other manufacturing machinery placed in a vehicle body assembly line is referred to as a robot position compensation method. The conventional robot position compensation method is divided into two sub-methods: a welding gun calibration method, in which the difference between CAD modeling data of a welding gun and information of an actual welding gun is compensated, and a layout calibration method, in which CAD modeling data of the positioning of robots and jigs and actual positioning of the robots and jigs is calibrated.
As an example of the welding gun calibration method, with reference to FIGS. 3 and 4, with a welding gun 53 mounted to an arm of a vehicle body panel welding robot 51, six axis joints of which are driven by a servo motor (not shown), production errors of the welding gun 53 and attachment errors of the robot 51 are compensated. In more detail, a needle pin 55, which has a sharp end and is made of steel, is first produced and is installed within a working radius of the robot 51 in step S100. Next, the user/operator teaches the robot 51 through a robot controller 61 such that that a lower tip 57 of the welding gun 53 is positioned at the end of the needle pin 55 in step S110.
Subsequently, in a state where the lower tip 57 of the welding gun 53 is positioned at the end of the needle pin 55, a reference point is designated and teaching of the robot 51 of at least four postures is performed in step S120. Next, teaching program data corresponding to the four or more positions is transmitted to a main computer 59 in step S130. At this time, the four or more positions of the robot 51 do not merge at a single point (i.e., the reference point) in the robot teaching program as a result of robot position errors (backlash). Accordingly, possibly four or more reference points result.
Following the above, the main computer 59 runs the uploaded robot teaching program, then compensates the four or more reference points to a single point in step S140. Next, an error between CAD data of a distance from a first axis at an end of the lower tip 57 of the welding gun 53 to a second axis, which is a connecting portion of the welding gun 53, and data modeled through a simulation is compared with a predetermined value. That is, in step S150, it is determined if the error between the CAD data and the simulation data is less than the predetermined value.
If the condition of step S150 is satisfied, position compensation of the welding gun 53 is completed, then the robot teaching program is downloaded to the robot controller 61 in step S160. However, if the condition of step S150 is not satisfied, welding gun data modeled through the simulation is revised in step S170, after which step S160 is performed, thereby completing the welding gun calibration method.
An example of the layout calibration method will now be described with reference to FIGS. 5 and 6. In a state where a welding gun 53 is mounted to an arm of a vehicle body panel welding robot 51, six axis joints which are driven by a servo motor (not shown), and jigs 65, which include a clamp, locator and a tooling pin for controlling a vehicle body panel 63, are installed according to car assembly line coordinates, errors in the positioning of the robot 51 and the jigs 65 are compensated. This will be described in more detail below.
First, a distance T1 is measured using a tape measure to determine a position of the robot 51 and a distance T2 is determined from a robot product drawing, after which a robot position is calculated in step S200. Next, a dowel pin 67, which is made of steel and has a sharp end is inserted in a NC hole to connect one jig 65 to another, is manufactured and installed in one of the jigs in step S210. The user/operator then teaches the robot 51 through a robot controller 61 such that that a lower tip 57 of the welding gun 53 is positioned at the end of the dowel pin 67. In this state, a reference point is designated and teaching of the robot 51 to at least four postures is performed in step S220.
Following the above, teaching program data corresponding to the four or more positions is transmitted to a main computer 59 in step S230. The main computer 59 then runs the uploaded robot teaching program, and considering the four or more reference points that do not merge at a single point, makes a comparison of data modeled through simulation such that robot position data are compensated to correspond to actual positions in step S240. Next, after position compensation of the welding gun 53 is completed, the robot teaching program is downloaded to the robot controller 61 in step S250, thereby concluding the layout calibration method.
However, in the conventional robot position compensation method as described above, manufacturing time is increased as a result of the method being divided into two sub-methods that are separately performed, that is the welding gun calibration method, in which the difference between CAD modeling data of the welding gun and information of the actual welding gun is compensated, and the layout calibration method, in which CAD modeling data of the positioning of robots and jigs and actual positioning of the robots and jigs is calibrated. Also, errors are common in the actual measuring of the robot position such that a second compensation procedure performed through operator teaching is required.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a method for compensating the position of a robot in which a laser measuring instrument is used to simultaneously compensate for the positioning of a welding gun, a welding robot, various jigs, etc. such that a robot teaching process time is reduced and a precision of welding point teaching for a vehicle body panel robot is enhanced.
To achieve the above object, the present invention provides a method for compensating the position of a robot using a laser measuring instrument. The method comprises the steps of (a) establishing an origin coordinate system by installing a reflector in each of a plurality of jig NC holes of a jig table, precise locations of which are known based on a car assembly line coordinate system, irradiating laser beams onto the reflectors by the laser measuring instrument and calculating distances to the reflectors based on properties of the laser beams reflected back to a sensor head of the laser measuring instrument, setting one of the NC holes of the jig table as an origin, with lines extending from the origin to two other NC holes as X and Y axes, and with a line normal to the plane formed by the three NC holes as a Z axis; (b) converting, in a controller of the laser measuring instrument, the origin coordinate system to an established coordinate system to enable the laser measuring instrument to recognize the origin coordinate system as a car assembly line coordinate system; (c) generating coordinates of an end of a lower tip of a welding gun by irradiating a laser beam onto a reflector installed on the end of the lower tip of the welding gun and calculating a distance to the reflector based on the properties of the laser beam reflected back to the sensor head of the laser measuring instrument; (d) calculating absolute coordinates of the end of the lower tip of the welding gun by using the coordinates determined in steps (a) and (c) to calculate a distance between the NC holes and the end of the lower tip of the welding gun, and calculating a position and posture of the robot using the absolute coordinates; (e) teaching the robot four or more postures by setting the end of the lower tip of the welding gun as a reference point; (f) uploading position coordinates of the robot calculated in step (d) and robot teaching program data of step (e) to a main computer; (g) performing error compensation by the main computer in which the main computer runs the uploaded robot teaching program and compensates the four or more points not converging at a single point so that the points merge at the reference point, which is at the end of lower tip of the welding gun, and compares the points with data modeled through simulation in consideration of the plurality of points of the jig so that the points are made to correspond to an actual position standard; (h) determining if an error between CAD data corresponding to the distance to the end of the lower tip of the welding gun and data modeled through simulation is less than a predetermined value; and (i) completing compensation, if the error between the CAD data of the distance to the end of the lower tip of the welding gun and the data modeled through simulation is less than the predetermined value, of positions of the welding gun, the robot and the jigs, and downloading the robot teaching program to a robot controller.
According to a feature of the present invention, in step (h), if the error between the CAD data of the distance to the end of the lower tip of the welding gun and the data modeled through simulation is greater than or equal to the predetermined value, welding gun data modeled through simulation is revised, then step (i) is performed.